1. Technical Field
The system and method described herein relates to monitoring the average optical power output and the extinction ratio of a point light source.
2. Discussion of the Related Art
Optical telecommunication systems include the use of point light sources, e.g., lasers, to transmit information at high speeds through optical fibers. The threshold current and slope efficiency of typical point light sources vary due to age and changes in operating temperature. In order to control the average optical power output of the point light sources, photodetectors are placed in a control feedback loop to monitor the optical output of the laser. If the signal received by the photodiode should fall, for example, the electrical current supplied to the laser would be increased to compensate.
Point light source and monitor photodetector combinations may be mounted in a specially designed package which has a mounting base with insulated connector leads and a sealed cover. The cover may include a window of glass, or other transparent material over a central portion of the top such that the window is aligned with the emitting aperture of the point light source device. In some point light source and photodetector combinations, reflected light from the window of the glass is received by the photodetector. Because the light fluence or power in these systems is generally small and unfocused, large photodiodes are needed to gather enough light to provide a sufficient signal-to-noise ratio (SNR) to maintain the constant average optical output from the laser. Unfortunately, large area detectors have low electrical bandwidth, making them unsuitable for tracking the high speed modulation of the laser. Instead, they are limited to use as time-average power monitors.
Changes in the slope efficiency of the laser with temperature and age also affect the extinction ratio of the point light source output. The extinction ratio of a point light source is the optical power of the “one” state divided by the optical power of the zero state. In systems employing large area monitor photodetectors, the change in extinction ratio is generally ignored or corrected using a look-up table based on data obtained by characterizing lasers similar to those used in the system of interest. Alternatively, the superposition of a pilot tone, at a frequency within the bandwidth of the monitor photodetector, onto the data may be used to correct changes in the extinction ratio of the point light source. This approach, based on the principle that the amplitude of the received pilot tone is proportional to the amplitude of the data modulation, has the drawback of modulating the extinction ratio of the transmitted data as well, thereby introducing extra noise.